The present invention relates to implantable hearing systems for assisting hearing in hearing impaired persons.
Some implantable hearing assistance systems use a microphone located in or near the ear to convert acoustic sound energy into an electrical signal. The electric signal is amplified, modulated and then directly communicated by a transducer to the inner ear to stimulate the cochlea to assist hearing. Alternatively, the amplified signal is communicated to a transducer for conversion to mechanical acoustic energy for vibratory application to the stapes of the middle ear or the cochlea. The mircrophone can be located externally, subdermally adjacent the ear, or within the external auditory canal. The transducer is commonly connected to a portion of the middle ear, known as the ossicular chain, which includes the malleus, incus and stapes. Vibrations are emitted from the transducer into and through the ossicular chain to the cochlea of the inner ear.
The ossicular chain facilitates forward transmission of mechanical sound vibrations from the tympanic membrane of the external auditory canal to the inner ear. However, the ossicular chain also permits reverse transmission of mechanical sound energy to be transmitted from the transducer of the implantable hearing assistance system, back through the ossicular chain to the tympanic membrane, and into the external auditory canal. This retrograde sound transmission passes out of the external auditory canal and is acoustically fed back to the microphone of the system.
This acoustic feedback limits the maximum gain which the hearing assistance system can apply to the signal received by the microphone. In particular, the feedback created by reverse bone conduction through the ossicular chain has an inverse relationship with usable gain. For example, if one percent of the acoustic vibratory signal emitted by the transducer to the stapes, or other part of the ossicular chain, is fed back through the ossicular chain and into the external auditory canal to the microphone, the gain for the hearing assistance system is limited to roughly 100 or 40 dB. Due to the nature of the hearing losses and the acoustic limitations of these systems, a much higher gain is ideal. Accordingly, reduction or elimination of this feedback is desirable.
Moreover, these hearing assistance systems, which transmit acoustic sound energy onto an ossicular chain with a transducer, are inefficient and consume power rapidly. Inefficiency results from the mechanical force that must be exerted by the transducer against the ossicular chain. This inefficiency causes rapid power consumption, requiring frequent battery changes. Battery changes are, at least, inconvenient for an externally located battery, and at worst, costly and surgically-related for a battery implanted in the middle ear or subdermally.
The importance of restoring hearing to hearing impaired persons demands more optimal solutions in hearing assistance systems. Ideally, an improved hearing assistance system both minimizes power consumption as well as maximizes gain to produce a better acoustic signal for reception into the cochlea and the inner ear.
A method and apparatus of the present invention improves hearing for a hearing impaired person by introducing and maintaining a mechanical feedback barrier between a microphone and a transducer of an implantable hearing assistance system. In this method, mechanical sound vibrations impinging on the person""s body habitus are received with an electromechanical device (e.g. microphone) disposed at a body habitus sound reception site. The body habitus sound reception site can be located within the external auditory canal, or external of the external auditory canal either subdermally or external of the scalp, or even subdermally along the external auditory canal.
The mechanical sound vibrations are converted with the electromechanical device to an amplified electrical signal. Next, the amplified electrical signal is delivered to the inner ear with a transducer operatively coupled between the electromechanical device and the middle ear or the inner ear.
Finally, a mechanical feedback barrier is introduced and maintained between the sound reception site and the transducer to minimize acoustic feedback therebetween. Preferably, this feedback barrier is established by removing a portion of the hearing impaired person""s ossicular chain (e.g. malleus, incus, or stapes) or fixing a portion of the ossicular chain to prevent transmission of sound feedback. In other embodiments, a portion of the ossicular chain is not removed but merely separated so that the procedure can be reversed if desired at a later time.
This method and apparatus of the present invention optimizes hearing improvement by preventing unnecessary mechanical feedback that can occur through the ossicular chain and the external auditory canal. Interrupting the ossicular chain, or otherwise immobilizing the ossicular chain, to prevent this retrograde sound transmission permits significant enhancement of the gain applied to the amplified electrical signal transmitted to the stapes or inner ear. In addition, less mechanical energy is required to transmit the acoustic energy to the interrupted ossicular chain or cochlea than when the ossicular chain remains intact. Accordingly, this method and apparatus reduces power consumption and frequent battery replacement for implantable hearing assistance systems.